Tree-ring growth recovers, but δC and δN do not change, after the removal of point-source air pollution: a case study for poplar ( Populus cathayana) in northwestern China.

Pollution from urban centers and fossil fuel combustion can decrease forest growth and interfere with physiological processes. To evaluate whether tree growth and the carbon isotope ratio (δC) and nitrogen isotope ratio (δN) in tree rings can serve as proxies for air pollution, this study compared these indices for poplar ( Populus cathayana) growing at urban and suburban locations in Lanzhou, in northwestern China. Basal area increment values were much lower at the urban site than in the suburbs from 1985 to 2009, were negatively correlated with NO ( r = −0.56, p < 0.01) and SO ( r = −0.52, p < 0.05) emissions from 1990 to 2009, and increased abruptly after the Lanzhou urban steel factory closed. Urban tree-ring δC values were not significantly correlated with NO and SO concentrations, and did not differ significantly between the two sites, indicating that other environmental effects (such as precipitation) masked the pollution effects. Tree-ring δN values in the urban samples were much higher than the suburban values. Such differences may be attributable to uptake of N-enriched compounds caused by a higher urban N deposition rate. Tree growth is a promising tool for detecting ecophysiological responses of trees to both diffuse and point-source air pollution, but δC and δN in poplar were not sensitive to point-source air pollution in a heavily polluted environment. [ABSTRACT FROM AUTHOR]